Recent developments of 3GPP Long term evolution (LTE) facilitate accessing local Internet protocol (IP) based services in the home, office, public hot spot or even outdoor environments. Local IP access and local connectivity involving direct communication between devices in the close proximity of each other, has gained considerable interest. Close proximity is here typically less than a few tens of meters, but sometimes up to a few hundreds of meters.
This direct or device-to-device (D2D) mode of communication enables a number of potential gains over traditional cellular techniques, because D2D devices are typically much closer to one another than cellular devices having to communicate via cellular access points, such as eNodeBs, are.
Two potential gains, capacity gain and peak rate gain are noted. Since radio resources, e.g. Orthogonal frequency division multiplexing (OFDM) resource blocks, between the D2D layer and cellular layer may be reused, a reuse gain can be obtained. Due to the proximity and potentially favourable propagation conditions high peak rates can be achieved, for which reason a proximity gain can be obtained.
In addition, direct D2D communication allows UEs to sense their proximity, enabling a number of proximity-based services and applications.
Recently, device-to-device (D2D) communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. It has been suggested that device-to-device communication can share the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes.
Allocating dedicated spectrum for device-to-device purposes is however a less likely alternative as spectrum is a scarce resource. Sharing between device-to-device services and cellular services is more flexible and provides higher spectrum efficiency than using dedicated spectrum.
Devices wanting to communicate, or even just discover each other, typically need to transmit control signalling. One example of such control signalling is the so-called beacon or discovery signal, which at least carries some form of identity of the device transmitting the signal and which is transmitted by a device wanting to be discoverable by other devices. Other devices may scan for the beacon signal, and once they have detected the beacon signal, they can take the appropriate action, for example to try to initiate a connection setup with the device transmitting the beacon signal.
For certain communication modes, e.g., connectionless communication, typically employed for groupcast and broadcast transmission, the beacon signal might carry a scheduling assignment indicating an associated data transmission to potential receivers. Connectionless communication is typically a unidirectional communication mode that does not require acknowledged connection setup.
Communication may be classified in different types being unicast, broadcast, and groupcast/multicast. Unicast is used to describe communication where a piece of information is sent from one point, i.e. a single source, to another point being a specified destination. In this case there is just one sender, and one receiver.
Communication where a single device is transmitting a message to all other devices in a given address range, is described by broadcast.
Groupcast/Multicast is used to describe communication where a piece of information is sent from one or more points to a set of other points. In this case there may be one or more senders, and information is distributed to a set of receivers. The set may however comprise no receivers or any other number of receivers.
The format of multicast packets is identical to that of unicast packets and is distinguished only by the use of a special class of destination address. Unlike broadcast transmission, multicast clients receive a stream of packets only if they have previously elected to do so, for example by joining the specific multicast group address.
A straightforward way to perform D2D groupcasting is to let a UE broadcast via a data channel to a group of UEs. The broadcasting UE may be under network coverage, but it is not a necessity. The UEs of the group of UEs which are within radio proximity of the transmitter will receive the transmission. A clear disadvantage is that UEs of the group which are not in radio proximity of the transmitter will accordingly not be reached.
There is hence a need for a solution addressing one or more of these issues as discussed above.